Bone fixation assembly and method

ABSTRACT

A bone fixation assembly and method of fixing bones including a fixation element having a head portion, a unitary coupling element and at least one locking element. The unitary coupling element includes a first bore adapted to slidably receive the head portion of the fixation element. A first locking element is adapted to secure the head portion in the first bore.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of French Application No.0311291 filed on Sep. 26, 2003, the contents of which are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to bone fixation devices, and in particular, topedicle fixation assemblies and methods used in spinal fixationprocedures.

BACKGROUND OF THE INVENTION

The spinal column is a highly complex system of bones and connectivetissues that provides support for the body and protects the delicatespinal cord and nerves. The spinal column includes a series of stackedvertebral bodies, each vertebral body including an inner or centralportion of relatively weak cancellous bone and an outer portion ofrelatively strong cortical bone. Situated between each vertebral body isan intervertebral disc that cushions and dampens compressive forcesexerted upon the spinal column. A vertebral canal containing the spinalcord and nerves is located behind the vertebral bodies.

A surgical technique commonly referred to as spinal fixation usessurgical implants for fusing together and/or mechanically immobilizingtwo or more vertebral bodies of the spinal column. Spinal fixation mayalso be used to alter the alignment of adjacent vertebral bodiesrelative to one another to change the overall alignment of the spinalcolumn. Such techniques have been used effectively to treat a widevariety of conditions and, in most cases, to relieve pain.

One spinal fixation technique involves immobilizing the spine usingorthopedic stabilizing rods, commonly referred to as spine rods, whichrun generally parallel to the spine. This technique involves exposingthe spine posteriorly and fastening bone screws to the pedicles ofvertebral bodies. The pedicle screws are generally placed at least oneper vertebra and serve as anchor points for the spine rods. Clampingelements adapted for receiving a spine rod therethrough are then used tojoin the spine rods to the pedicle screws. The aligning influence of thespine rods forces the spinal column to conform to a more desirableshape. In certain instances, the spine rods may be bent to achieve thedesired curvature of the spinal column.

Most existing rod fixation systems require several components to buildthe systems. Each additional component or instrument required toassemble the fixation system adds to the complexity of the surgicaltechnique. A need has thus arisen for improved fixation systems thatminimize the assembly of small pieces of hardware during the surgicalprocedure. Thus, there remains a need for spinal fixation devices thatfacilitate simple and fast assembly of attachment of a spinal rod to aspine. It would be desirable to provide a device with pre-assembledcomponents that will result in less time in assembling the components inthe operating room.

SUMMARY OF THE INVENTION

In accordance with one or more embodiments of the present invention, abone fixation assembly is provided comprising a fixation element havinga head portion, a unitary coupling element having a first bore adaptedto slidably receive the head portion of the fixation element, and afirst locking element adapted to secure the head portion in the firstbore. In certain embodiments, the coupling element has a second boreadapted to receive a spinal rod and a second locking element adapted tosecure the spinal rod in the second bore.

According to one or more further aspects of the present invention, abone fixation system assembly includes a fixation element having asubstantially cylindrical head portion; a unitary coupling elementhaving a first bore adapted to receive the head portion of the fixationelement and to permit axial movement of the coupling element withrespect to the fixation element, the coupling element having a secondbore adapted to receive a spinal rod; a first locking elementpre-assembled with the coupling member and adapted to secure the headportion in the first bore; and a second locking element pre-assembledwith the coupling member and adapted to secure the spinal rod in thesecond bore.

According to one or more further aspects of the present invention, abone fixation assembly includes: a fixation element having a headportion; a unitary coupling element having a first bore adapted toreceive the head portion of the fixation element and to permit axialmovement of the coupling element with respect to the fixation element;and a locking element including a ball ring and a locking nut adapted toexert radial force on the ball ring such that the ball ring exerts acompressive force on the head of the fixation element to secure the headof the screw in the coupling element.

According to still one or more further aspects of the present invention,a bone fixation assembly includes: a fixation element having asubstantially cylindrical head portion; a unitary coupling elementhaving a first bore adapted to receive the head portion of the fixationelement and to permit axial, preferably polyaxial, movement of thecoupling element with respect to the fixation element, and a firstlocking element including a locking nut that engages the first bore anda tapered opening adapted to allow polyaxial motion of the head of afixation element inserted therethrough.

In still another embodiment of the invention, a method of fixing a bonein place is provided. The method comprises attaching a fixation elementhaving a head portion to a bone, sliding a first bore of a unitarycoupling element over the head portion, inserting a rod through a secondbore of the coupling element, tightening a first locking elementassociated with the first bore to secure the head portion to thecoupling element, and tightening a second locking element associatedwith the second bore.

According to one or more embodiments of the present invention, bonefixation assemblies are provided to connect spinal rods without anyloose parts and very few steps (preferably only two steps) to lock thefixation element to the coupling member and the spinal rod to thecoupling member. The assemblies of the present invention do not requireany additional locking mechanism, and they reduce the assembly of smallpieces of hardware during the surgical procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the subject matter of the presentinvention and the various advantages thereof can be realized byreference to the following detailed description in which reference ismade to the accompanying drawings in which:

FIG. 1 is a partial perspective view of a bone fixation assembly with aspine rod secured thereto according to one or more embodiments of thepresent invention;

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is a sectional view of a bone fixation assembly with a spine rodand screw illustrating the manner in which the assembly is connectedwith a vertebra;

FIG. 4 is a top perspective view of a coupling element according to oneor more embodiments of the present invention;

FIG. 5 is a side elevational view of a locking element for securing aspine rod to a bone fixation assembly according to one or moreembodiments of the present invention;

FIG. 6 is a top view of the locking element shown in FIG. 5;

FIG. 7 is a perspective view of a locking nut for securing the head of afixation element according to one or more embodiments of the invention;

FIG. 8 is a cross-sectional view of the locking nut taken along line 8-8in FIG. 7; and

FIG. 9 is a perspective view of a ball ring according to one or moreembodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orcarried out in various ways.

Referring now to the drawings and particularly to FIGS. 1-3, a bonefixation assembly 10, in accordance with certain preferred embodimentsof the present invention, is shown. The bone fixation assembly may besecured to the pedicles 11 of vertebral bodies of a spinal column, asshown in FIG. 3. The fixation assembly includes a coupling element 12,preferably made of a biologically inert material, for example, any metalcustomarily used for surgical devices and particularly those used forbone screws and pins, such as titanium or stainless steel. Othersuitable materials for the coupling element include, but are not limitedto, alloys, composite materials, ceramics or carbon fiber materials.

The fixation assembly 10 includes a fixation element 16 having a headportion 18. The head portion 18 is preferably cylindrical, andpreferably has a smooth outer surface to facilitate sliding on or inconnection with the head portion 18, as will be described below. Thehead portion 18 can be constructed in other ways or in connection withother components to facilitate sliding of the coupling element withrespect to the fixation element 16. The fixation element 16 could be anysuitable fixation element for attachment to a bone, for example, a hookor a screw. In preferred embodiments, the fixation element includes ascrew fastener having the head portion 18 and a threaded portion 20, andthe threads are adapted to be engaged in bone material. The end of thescrew fastener opposite the head portion 18 has a tip for insertion intobone, and external screw threads 20 extend between the tip and the headportion 18. The screw threads 20 have an inner diameter and an outerdiameter. The fixation element, including the screw threads 20 and headportion 18, are preferably made of a biologically inert material, suchas titanium or stainless steel.

In the embodiment shown in FIGS. 1-3, one end of the head portion of thefixation element includes a tool engagement surface 22. In theembodiment shown, the tool engagement surface 22 is in the form of ahollow female hex head adapted to receive an end of a hexagonal driverfor turning the fixation element. It will be understood, however, thatother internal or external tool engagement surfaces 22 can be usedaccording to the present invention.

Coupling element 12, which is shown in more detail in FIG. 4, preferablycomprises a unitary body and has at least a first bore 14 having alongitudinal axis adapted to receive the head portion 18 of the fixationelement 16 and to permit axial, sliding movement of the coupling element12 along the axis of the head portion 28 18 with respect to the fixationelement. The fixation assembly further comprises a first locking elementadapted to secure the head portion 18 in the first bore 14. According toone or more embodiments, the first locking element includes at least alocking nut 24.

Further details on the locking nut 24 are shown in FIGS. 7 and 8.Preferably, the locking nut 24 is hollow and has a bore 26 therethroughfor receiving the head portion 18 of the fixation element 16. Thelocking nut 24 includes a receiving end 28, and the opening 26 at thereceiving end 28 is flared or angled as shown in FIG. 8. The flared ortapered opening permits polyaxial motion of a fixation element insertedtherethrough. In a preferred embodiment, the first bore 14 includes abottom portion 15 that is tapered or flared in a direction opposite thetaper or flared opening in the locking nut 24. The oppositely extendedflared openings permit polyaxial movement between the fixation elementand the coupling member. Preferably, the fixation element can movepolyaxially as shown by the arrows “a” in FIG. 2. In preferredembodiments, the fixation element can move by at least 20 degreespolyaxially, as indicated by the arrows “a” in FIG. 2. The locking nut24 further includes a seating end 30, and according to at least oneembodiment, the seating end 30 includes an inner curved engagementsurface 32 for engaging a ball ring, which will be described in moredetail below. The ball ring, as seated in the inner curved engagementsurface 32, facilitates polyaxial movement of the fixation element, andcan do so in a controlled manner. The receiving end 28 of the lockingnut may further include a plurality of flat surfaces 34 such that thereceiving end 28 of the locking nut is hexagonally shaped and adapted toreceive a wrench or other device adapted to turn the locking nut.According to certain preferred embodiments, the exterior surface of thereceiving end 28 further includes male threads for engagement withcomplementary female threads on the interior of the first bore 14 of thecoupling element 12.

According to one or more embodiments, the coupling element 12 includes asecond bore 36 adapted to provide an opening to receive a spinal rod 38through the bore. The second bore 36 has a longitudinal axis that issubstantially transverse to the longitudinal axis of the first bore 14.In these embodiments, a second locking element 40 is provided and isadapted to secure the spinal rod in the second bore. FIGS. 5 and 6 showadditional details of the second locking element 40. The second lockingelement 40 is preferably in the form of a set screw. The second lockingelement 40 includes a hexagonal-shaped opening which is adapted toreceive an end of a hexagonal driver for turning the second lockingelement 40. The second locking element 40 further preferably includesexternal threads 42 for engagement with complementary internal 43threads on the surface of the second bore 36. The rod receiving openingand second locking element are thus adapted to capture and seat anorthopedic stabilizing rod therein. In preferred embodiments, both thefirst and second locking elements 22, 40 are permanently seated in thecoupling element 12 such that the locking elements cannot beinadvertently removed from the coupling element. Inadvertent removal ofthe locking element 40 can be accomplished by providing a flared portionor lip 41 on the end of the locking element opposite the tool engagementsurface. A shoulder 45 associated with the respective bore of thecoupling element prevents removal of the locking element 40. The flaredportion or lip 41 has a diameter that is greater than the threadedportion of the locking element 40. The shoulder 45 associated with thebore 36 provides an opening in the bore 36 that is greater than thediameter of the threaded portion of the locking element 40, but smallerthan the diameter of the flared portion or lip 41 on the locking element40. Thus, when the locking element is turned in a direction to back thelocking element out of the bore 36, the shoulder 45 and flared portionor lip 41 prevent the locking element 40 from coming out of the bore 36.Although the shoulder 45 and flared portion or lip 41 are shown withrespect to the second bore 36, it will be understood that such astructure can be used with respect to the first bore 14 to preventinadvertent removal of the locking nut 24 from the first bore. It willbe understood, of course, that other means and structures can be used toprevent inadvertent removal or pre-assembly of the locking element 40and locking nut 24 in their respective bores. For example, the lockingelement and/or locking nut could be press fit into the bores andfriction-locked to prevent inadvertent removal. Alternatively, detentson the surfaces of the bores or the locking nut and locking elementcould be utilized to prevent inadvertent removal of the locking elementor locking nut.

According to preferred embodiments of the invention, the first lockingelement comprises a locking nut 24 that cooperates with a ball ring 44to lock the head portion 18 of the fixation element to the couplingmember 14. As shown in FIG. 2, the ball ring 44 is seated within thefirst bore 14 and at least a portion of the locking nut 24circumferentially surrounds a portion of the ball ring. In preferredembodiments, the ball ring 44 is pre-assembled and permanently seated inthe first bore of the coupling member. In preferred embodiments, thecurved engagement surface of the locking nut 24 engages the exteriorsurface 46 of the ball ring 44. FIG. 9 shows a detailed view of the ballring. The ball ring 44 shown in FIG. 9 comprises a generally sphericalouter surface having an opening 48 through the body coaxial with thefirst bore and adapted to receive the head portion 18 of the fixationelement 16. As shown in FIG. 9, the ball ring 44 includes a gap 50 inthe outer surface, allowing the ball ring 44 to be compressed such thatthe inner diameter of the ball ring opening 48 is reduced when the ballring is compressed. The ball ring 44 and the gap 50 are designed suchthat when the ball ring 44 is compressed, the diameter of the opening 48is less than the diameter of the head portion 18 of the screw so thatthe head portion of the screw is securely and snugly held in place.Compression of the ball ring occurs when the locking nut 24 istightened. Engagement of the male threads of the locking nut with thefemale threads in the first bore causes the engagement surface 32 of thelocking nut to engage the outer surface 46 of the ball ring 44, causingthe ball ring to compress onto the head portion 18 of the screw. Thiscompression exerts a radial force on the exterior surface of the ballring 44. It is to be understood that the ball ring 44 may be configureddifferently than shown in FIG. 9. For example, instead of a single gap50 in the outer surface of the ring, the ring may include a plurality ofsplit openings that do not extend through the entire exterior surface ofthe split ring.

Additionally, while the foregoing preferred structures facilitate axialmovement of the coupling element, other structures are also availablefor this purpose. For example, a split collet could surround a sphericalhead portion and the outer surface of the collet could cooperate withthe coupling element or other components to permit axial sliding beforebeing compressed around the spherical head portion. Of course, in such aconstruction, the extent of axial sliding may be limited by the size ofthe coupling element, whereas in the preferred embodiment, the extent ofsliding may be limited by the length of the head portion of the fixationelement.

According to a preferred embodiment, a fixation assembly 10 is providedin a package including the coupling element 12 having the set screw 40,locking nut 24 and ball ring 44 permanently pre-seated in the couplingelement 12 to reduce the number of loose parts and prevent any smallloose parts from being lost, or from having to be handled andmanipulated during surgery, the ball ring 44 is generally kept in theinterior of the coupling element by virtue of the nut 24 being capturedin connection with the coupling element. As used herein, the terminology“permanently pre-seated” means that the elements are prevented frombeing inadvertently removed from their respective bores, as opposed tobeing loose in a package and requiring assembly of the individualcomponents. Such a pre-seated construction helps to prevent the surgeonfrom losing or dropping small loose parts during preparation for asurgical procedure.

In use, the components are removed from the package, and a fixationelement such as a screw 16 having a generally smooth, cylindrical headportion 18 is inserted into the first bore 14 of the coupling elementand through the coaxial openings in the ball ring 44 and locking nut 24.A spinal rod 38 is inserted in the second bore of the coupling member.The set screw 40 may be tightened to exert force on the spinal rod andlock the spinal rod in place. The fixation element 16 may then beinserted into a bone, preferably into a previously drilled pilot hole inthe bone. The fixation element is then preferably screwed into the boneusing a driver or other appropriate device, advancing the fixationelement along its longitudinal axis into the bone. Prior to tighteningthe locking nut 24, the angle of the fixation element is adjusted bymoving it in the tapered opening 26 in the locking nut and theoppositely tapered opening of the first bore 14. Due to the oppositetaper of the first bore 14 of the coupling element and the taper of theopening in the locking nut 24, the fixation assembly can be manipulatedto cover a broader range of angles for capturing an orthopedicstabilizing rod. The range of angles that can be covered is 20 degreespolyaxially about the head portion 18 of the fixation element. After theproper angulation is determined, the locking nut is tightened, exertinga compressive radial force on the ball ring and locking the head portionof the fixation element in place. Preferably, the connector is locked inthe manner described above in two steps. However, according to certainprocedures, the locking nut may be tightened on the head portion of thefixation element first, and then the set screw may be tightened on thespinal rod after the locking nut has been tightened. Achievingsufficient angulation between anchoring elements while engaging theorthopedic rod is essential for assemblies mounted in spines havingabnormal curvatures. Sufficient angulation is also important in thecervicothoracic junction of the spine.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. For example, the bores for receiving the spinal rodsor heads of the fixation elements may have non-circular cross-sectionalshapes, such as square, pentagonal, elliptical, etc. It is therefore tobe understood that numerous modifications may be made to theillustrative embodiments and that other arrangements may be devisedwithout departing from the spirit and scope of the present invention asdefined by the appended claims and their equivalents.

The invention claimed is:
 1. A bone fixation assembly comprising: aunitary coupling element having a first bore having an axis, the firstbore adapted to receive a head portion of a fixation element and topermit axial, sliding movement of the coupling element relative to thefixation element, the coupling element having a second bore adapted toreceive a spinal rod; a first locking element adapted to secure the headportion of the fixation element in the first bore at any point along thelength of the head portion of the fixation element, wherein the firstlocking element comprises a consists essentially of an threadedcompressible ball ring; and a second locking element adapted to securethe spinal rod in the second bore.
 2. The bone fixation assembly ofclaim 1, wherein the first locking element further comprises comprisinga locking nut for engaging the ball ring.
 3. The bone fixation assemblyof claim 2, wherein the locking nut contains external male threads thatengage female threads formed in the first bore.
 4. The bone fixationassembly of claim 3, wherein the compressible ball ring is seated withinthe first bore and at least a portion of the locking nutcircumferentially surrounds a portion of the ball ring.
 5. The bonefixation assembly of claim 4, wherein engagement of the male threads ofthe locking nut with the female threads in the first bore exerts radialforce on the compressible ball ring to secure the head portion of thefixation element in the first bore.
 6. The bone fixation assembly ofclaim 5, wherein the locking nut is in a locked position, and thecompressible ball ring is in contact with the coupling member and thelocking nut.
 7. The bone fixation assembly of claim 6, wherein thesecond locking element includes a set screw.
 8. The bone fixationassembly of claim 7, wherein the set screw is permanently seated in thecoupling member.
 9. The bone fixation assembly of claim 8, wherein thelocking nut and compressible ball ring are permanently seated in thefirst bore.
 10. The bone fixation assembly of claim 8, wherein thelocking nut cannot be removed from said coupling element afterconnection therewith.
 11. The bone fixation assembly of claim 10,wherein a portion of the first bore is tapered and the locking nutincludes a tapered opening to permit polyaxial motion between thefixation element and the connector.
 12. The bone fixation assembly ofclaim 11, wherein the taper in the first bore and the taper in thelocking nut extend in opposite directions.
 13. The bone fixationassembly of claim 12, wherein the fixation element includes a screw. 14.The bone fixation assembly of claim 1, wherein the compressible ballring is at least partially split.
 15. The bone fixation assembly ofclaim 1, wherein said second bore has an axis transverse to the axis ofthe first bore.
 16. The bone fixation assembly of claim 1, wherein saidsecond locking element is connected to said coupling element and cannotbe removed from said coupling element after connection therewith. 17.The bone fixation assembly of claim 16, wherein said second lockingelement includes a flared lip that cooperates with a shoulder associatedwith the coupling element to prevent inadvertent removal of the secondlocking element.
 18. The bone fixation assembly of claim 1, wherein thefirst locking element is connected to said coupling element and cannotbe removed from said coupling element after connection therewith. 19.The bone fixation assembly of claim 1, further comprising a fixationelement having a head portion, wherein the head portion of the fixationelement is inserted in the first bore.
 20. A bone fixation assemblycomprising: a unitary coupling element having a first bore adapted toslidingly receive a head portion of a fixation element, and a secondbore adapted to receive a spinal rod; a first locking elementpre-assembled with the coupling element and adapted to secure the headportion of the fixation element in the first bore at any point along thelength of the head portion of the fixation element, the first lockingelement including a consisting essentially of an unthreaded compressibleball ring; and a second locking element pre-assembled with the couplingelement and adapted to secure the spinal rod in the second bore.
 21. Thebone fixation assembly of claim 20, wherein the first bore permits axialmovement of the coupling element relative to the fixation element. 22.The bone fixation assembly of claim 21, wherein the first bore permitspolyaxial movement of the coupling element relative to the fixationelement.
 23. The bone fixation assembly of claim 22, wherein the firstbore has an axis, and the second bore has an axis transverse to thefirst bore.
 24. The bone fixation assembly of claim 21, wherein thefurther comprising a locking nut is adapted to exert radial force on theball ring.
 25. The bone fixation assembly of claim 20, wherein thecompressible ball ring is seated in the first bore and cooperates with alocking nut threaded in the first bore.
 26. The bone fixation assemblyof claim 20, further comprising a fixation element having a headportion, wherein the head portion of the fixation element is inserted inthe first bore.
 27. The bone fixation assembly of claim 20, wherein saidcompressible ball ring is at least partially split.
 28. A bone fixationassembly comprising: a unitary coupling element having a first boreadapted to slidingly receive a head portion of a fixation element and topermit axial movement of the coupling element relative to the fixationelement; and a locking element including a an unthreaded ball ring and alocking nut associated with the head portion of the fixation element toexert a radial force on the ball ring such that the ball ring exerts acompressive force on the head portion of the fixation element to securethe head portion of the fixation element in the coupling element at anypoint along the length of the head of the fixation element, theunthreaded ball ring including upper and lower surfaces connected by acurved outer surface, and not integrally connected to any otherstructure.
 29. The bone fixation assembly of claim 28, furthercomprising a fixation element having a head portion, wherein the headportion of the fixation element is inserted in the first bore.
 30. Thebone fixation assembly of claim 28, wherein said ball ring is at leastpartially split.
 31. A bone fixation assembly comprising: a unitarycoupling element having a first bore adapted to slidingly receive a headportion of a fixation element and to permit axial movement of thecoupling element relative to the fixation element; aan unthreadedcompressible ball ring seated in the first bore adapted to secure thehead of the fixation element to the coupling element when the ball ringis compressed at any point along the length of the head of the fixationelement, the unthreaded compressible ball ring including upper and lowersurfaces connected by a curved outer surface, and not integrallyconnected to any other structure; and means for exerting compressiveradial force on the ball ring.
 32. The bone fixation assembly of claim31, wherein the means for exerting compressive radial force includes alocking nut.
 33. The bone fixation assembly of claim 32, wherein thelocking nut contains external male threads adapted to engage internalfemale threads in the first bore.
 34. The bone fixation assembly ofclaim 31, further comprising a fixation element having a head portion,wherein the head portion of the fixation element is inserted in thefirst bore.
 35. The bone fixation assembly of claim 31, wherein thecompressible ball ring is at least partially split.
 36. A bone fixationassembly comprising: a unitary coupling element having a first boreadapted to slidingly receive a head portion of a fixation element and topermit axial movement of the coupling element relative to the fixationelement; and a first locking element including a locking nut thatengages the first bore and a tapered opening adapted to allow polyaxialmotion of the head portion of the fixation element insertedtherethrough, the first locking mechanism element further including a anunthreaded compressible ball ring for locking the head portion of thefixation element at any position along its length, the unthreadedcompressible ball ring including upper and lower surfaces connected by acurved outer surface, and not integrally connected to any otherstructure.
 37. The bone fixation assembly of claim 36, wherein thelocking nut cooperates with the compressible ball ring to exert force onthe head of the fixation element to lock the fixation element withrespect to the coupling element.
 38. The bone fixation assembly of claim36, wherein the compressible ball ring is at least partially split. 39.A bone fixation assembly comprising: a fixation element having asubstantially cylindrical, smooth head portion; a unitary couplingelement having a first bore adapted to slidingly receive the headportion of the fixation element and to permit axial, sliding movement ofthe coupling element relative to the fixation element, the couplingelement having a second bore adapted to receive a spinal rod; a firstlocking element including an externally threaded locking nut adapted tocooperate with threads in the first bore and exert radial compressiveforce on a an unthreaded compressible ball ring slidably mounted on thehead portion of the fixation element pre-seated in the first bore tosecure the head portion of the fixation element in the first bore at anypoint along the length of the head portion of the fixation element, thelocking nut permitting polyaxial motion of the fixation element, theunthreaded compressible ball ring including upper and lower surfacesconnected by a curved outer surface, and not integrally connected to anyother structure; and a second locking element pre-assembled with thecoupling member and adapted to secure the spinal rod in the second bore.40. The bone fixation assembly of claim 39, wherein the compressibleball ring is at least partially split.